High current radio frequency ion source



A ril 2, 1963 M. E. ABDELAZIZ 3,084,273

HIGH CURRENT RADIO FREQUENCY ION SOURCE Filed Sept. 19, 1961 I BY 0 "46 /8 2 /2 z'ametrz'cal Distance from flxz'fi 2 g Jiforney niteai List $584,273 HEGH CURRENT BABES FREQUENY EON SOURCE Mohamed E. Abdeiaziz, Naperviile, EL, assignor to the United States of America as represented by the United States Atomic Energy tjomrnission Filed Sept. 19, 1961, Ser. No. 139,317 Claims. (Cl. 313-231?) The present invention relates to ion sources and more particularly to high current radio frequency ion sources. The conventional high current radio frequency ion source employs a ceramic cylindrical envelope having a radio frequency field generated therein parallel to the longitudinal axis thereof. An anode and cathode are positioned in the ends of the envelope and excited with at DC. potential to extract ions formed by the radio frequency field along an axis parallel thereto. The ion beam current from this type of source is insuflicient to meet the requirements of particle accelerators which require higher beam currents. The primary reasons for the lack of performance in higher beam currents result from (a) a limitation of the maximum extraction potential to a value of approximately 36 kV., (12) a decrease in percentage of extractable protons through recombination to molecular ions, (c) inability to focus the protons if the extraction cathode is extended too far through the plasma, and (d) loss of ionization from secondary elec trons due to their random scattering at the anode and cathode ends of the plasma container.

It is therefore one object of this invention to provide a device able to produce high proton beam currents.

It is another object of this invention toprovide a radio frequency ion source having ion extraction transverse to the radio frequency field.

It is still another object of this invention to provide a radio frequency ion source where extraction is effected from a region with lower recombination of protons to molecular ions.

It is yet another obiect of this invention to provide a radio frequency ion source having an extraction potential greater than 30 lrv. and improved focusing of the extracted ion current beam.

Other objects will become more apparent as the detailed description proceeds.

In general the present invention comprises a Pyrex cylindrical plasma container having a radio frequency field therein parallel to the longitudinal axis thereof. Means are provided for the extraction of the ion beam current transverse to the radio frequency field from an area midway between the ends of the container about the longitudinal axis thereof.

More complete understanding of the invention will best be obtained from consideration of the enclosed drawing in which:

FIG. 1 is a graphical representation of the electron concentration in a plasma container along a diameter AA.

FIG. 2 is a cross section view of the ion source of the present invention.

FIG. 3 is a graphical representation showing variations in extracted beam currents with distance from the axis BB of the device of FIG. 2.

Reference is made to FIGURE 1 to explain the theory for the present invention. A cylindrical plasma container it) is shown having fiat ends 12 and 14. When the interior of the evacuated container to is supplied with a gas such as hydrogen at a low pressure and a radio frequency field is generated within the container to parallel to the axis thereof, the hydrogen gas will be ionized into electrons, protons, and molecular ions. The electrons are the result of processes of electron production in the plasma, including secondary emission from the wall surfaces of the container 1i and their number N is obtained in the steady state when equilibrium between electron production and losses is established.

The cloud of N electrons will oscillate from one end of container 10 tothe other according to the direction of the oscillating radio frequency field. When the N oscillating electrons hit one of the ends 12 or 14- of chamber 10, they produce 6N secondary electrons, where 6 is the secondary emission coeflicient presumably greater than unity. In the collision, the N primary electrons are lost and the 5N secondary eletcrons are accelerated to the other end of the container 16 by the reverse polarity of the radio frequency field. In their passage, to the other end, some of the 5N electrons will collide with gas molecules or wall sides of the container 19, since electrons will be emitted at all angles to the emitted surface. The remaining electrons will be equal to N by the time they reach the other end where they collide and emit another 5 cloud of electrons. This process will be continually repeated.

The curve 16 shows the electron density distribution existing in the container in for the equilibrium conditions recited supra. As can be readily seen, the electron density is a minimum at the walls of the tube and rises to a maximum at the longitudinal axis of the container 10. The shape of the curve 16 exists throughout the longitudinal length of the container it The curve 16 is represented by the equation n,=n J (x), where n is the concentration of electrons at a radial distance r from the longitudinal axis of the container 16, n is the concentration of electrons at the longitudinal axis (r=0) and J (x) is a Bessel function of the zero order. The greatest value that x can have in the Bessel function of zero order is 2.405 which is when r equals the radius of the container It Greater values of x than this result in negative values of n which are meaningless.

The distribution of the ion density, that is, proton and molecular ion density, necessarily follows that of the above described electron density.

Turning now to FIGURE 2, a radio frequency ion source is illustrated which utilizes the above described analysis. A Pyrex cylindrical container 18 having flat ends 20 and 22 is the plasma container. Midway be tween the ends 20 and 22 of the container 18, the wall of the container 18 is opened and extended outwards to form an inlet passage 24 into the container 18 for unionized gases. A circular nickel anode 26 is mounted and sealed with epoxy glue in the walls of the passage 24.

Diametrically opposite the inlet passage 24, the wall of the container 18 is opened and extended outwards to provide for the insertion of an annular aluminum extraction cathode 28, having a cylindrical exit canal 3h. The cathode 28 is shielded by a Pyrex-shield 32. The shield 32, cathode 28, and the extended walls of the container 18 are sealed to a stainless steel mounting plate 34 to give a gas tight seal.

A radio frequency coil 36 is wound around the exterior surface of container 18 along the longitudinal axis B-B thereof.

For operation of the ion source disclosed in FIGURE 2, an unionized hydrogen gas is fed through the passage 24 from a gas source (not shown) A radio frequency generator (not shown) applies a signal to coil 36, thereby causing a radio frequency field to be generated within the container 18 along the longitudinal axis B-B thereof. The radio frequency field ionizes the gas as hereinbefore described for FIGURE 1. A D.-C. generator (not shown) generates a voltage which is applied across the anode 26 and cathode 28, whereby the positive ions ex- -isting-in the ionized gas within container 18--areextracted via the exit canal 30 in a direction transverse to the radio frequency field.

For determining the-optimum geometryandopera ting conditions, notation must-be made to. the characteristic ion-density curve disclosed by the analysis'of-FIGURE 1, since thiscurve exists a for the device of EIGURE 2.

Since the ion-density--isgreatest-about-the longitudinal Y axis -BB of the container 18, theextraction'cathode 28 should'be mounted sothat the-exit canal'SB -of the cathode 28 and shield "32 is .close to the. longitudinal axis "-BB. FIGURE 3-illustrates the-increase inextracte-d -ioncurren-t as the cathode 28'and its-associated shield?32 -is so mounted. There is -a-- practical limit to the distance -which the cathodeZS and shield 32-may-be .moved towards-the longitudinal -axisB-B.- 'If they were'to be moved so that theyintercepted the axis,:-thendist-urbance of the motion of the electrons" which oscillate between ends 20 -and 22 due to the IadiO frequency'fieId-may occur, thereby causing deviation from their theoretical: distrihution with a resultant lower-ingot the ionization dencity. It hasheen determined that the closest distance which the cathode 28 andshield 32=may approach the -longitudinal axis B-B- without seriously aifecting-thebehavior of the plasma is. approximately A the diameter :of the container 18.

To extract the optimum percentage of protons in-the ion beam, the cathode 28 and shield- 32. is positioned ap- -proxinrately midway between the ends 20 and22-of the 4 container --1S. At this point the region from which the beam is extracted isgeometrically furthest: frorrr the in- =terior-surfaces of the container walls-where recombination of. protons to molecular ionsoccurs. Thus, the rnaxL murnratio of protons-tomolecular ions in the extracted beamzis assured.

It will-also be notedthat in the conventionalion source, with extraction parallel to the radio trequency' 'field, the radio-frequency field and the D.'-C. extraction field are additive during-atpontion of a cycle of the-radio frequency field. These two additivefieldsgreatly enhance the possibility of arcing etween the anodeandextractioncathode and thereby limit the extraction potential to approxi- -rnately 30-kv. In thepresentinvention, the radio-frequency field and the Dr'C. extraction-field-a-re=mutually perpendicular and thus nonadditive. Further, the radio frequency-field tends to distort and hence blow any are v which may occur between the anode 26 and cathode 28. This field arrangement allows an extraction potential of approximately-40 kv. -to-be applied between the *anode "26 and cathode ZS without arcing taking-place. "The ability to apply a higher extractioirpotential to the ion source of FIGURE 2 permits. better focusing of the :extracted .ion beam than heretofore possible.

When. operated at a30 kv. extraction potential (maximum. for the conventional radio frequency ion source) the ion source of FIGURE 2 gives anion current beam whose. density -is.25 'to.30% higher than that of .the con- .ventional source .and which containsaprotonpercentage ,of 90% as opposed-to 85% for the conventionalesource. Further, as the extraction potential is increased to 40kv. -(not; possible without danger of'arc-ingin the conventional-ion source) the ion current "density'increaseslj over that at 30 kv. with the proton percentagerernain- .;ing.,the. same.

.Persons skillcdin the .art wil1,.of.course, readilyadapt the teachings of the invention to.- embodiments; far. differ- --ent -than the embodiment illustrated. Accordingly, the scope of protection afforded the invention should not be limited to the particular embodiment thereof illustrated in the drawings and described above, but shall be determined .onlyin. accordance with the. appendedclairns.

What is claimed is: *1. A radio frequency ion source ..comprising a cylindrical plasma container, means for injectingan unionized gas into said container, means for generating a radio frequency field withinlsaidcontainer parallelto the longitudinal axis thereof, ,said. radio frequency field ionizing said gas, and means for extracting transverse to said radio 3 frequency field the positive ions created by said radio fre- ;2.,'A.radio frequency ion source comprising a cylin- ;drical plasma container, means' forinjecting-an unionized gas .intosaid container, means'for generating a radio fre- J quencyfieldwithinsaid container parallel to the longi- :tudinal axis thereof, said radiofrequency field-ionizing :said gas, and means for extracting transverse to said radio :frequency tfieldfrom an.area= midway between the ends -:.of said container along the longitudinal axis thereof the ions created by. said radio frequency field.

3. A radio frequency ion source comprising a cylin- 25.drical.plasm,a container, means for injecting an unionized. gas intonsaid container, means for generating a radio (frequency fieldiwithin said container. parallel to the axis thereof, .said radio frequency fieldlionizing said gas, an

:anode monntedm-idway between the ends ofv said con- :.tainer and having. access to the interior surface thereof,

a cathode mounted diametrically opposite said anode and :.projecting throughthe Walls of said container to approximately the longitudinal axis of said container,-.and means :for..-applying .aiD.-C. potential across .said .anode. and cathode, .said cathode .extractingthe positive ions created byisaid radio frequency field.

.4. A radiofrequency; ion source comprising a Pyrex cylindrical plasma container,..a. coil disposed. around the .rexterior surface :ofsaidcontaine-r.along. the longitudinal wide thereof, means for. applying a radio-frequency signal :10 .said-ucoil means for injecting an unionized gasqinto said container, an anode mounted midwaybetween the goods ofnsaid;container and having ..access. to the interior surface .thereof,: a;cathode mounted diametricallyqopposite said, anode and: projecting through the walls. of said. container to a maximum of the diameterof said container, "and'meansforapplying a-D.-C. potential across said anode and cathode, said cathodecxtracting theions created:-by;said radio frequency field.

:5. 'A'radio'frequency ion source comprising aPyrex cylindrical container having-fiat ends, I a coil .disposed aroundthe exterior surface. of saidcontainermalongthe longitudinal axis; thereof, means for'applying a radio frequency.signal to said coil, means for injecting unionized hydrogen gas into said container,;an anode-mounted mid- Way' between the ends of said-containerand havingaccess -tothetinterior surface thereof, acathode mounted :di-

ametrically opposite said anode and projecting-through .the walls of said containera distance of 'ii the diameter --of;said container, and means for applying a40 kv- D.-C.

potential across said anode and cathode, said cathode :extracting the ionscreated by'said radio frequency field. 

1. A RADIO FREQUENCY ION SOURCE COMPRISING A CYLINDRICAL PLASMA CONTAINER, MEANS FOR INJECTING AN UNIONIZED GAS INTO SAID CONTAINER, MEANS FOR GENERATING A RADIO FREQUENCY FIELD WITHIN SAID CONTAINER PARALLEL TO THE LONGITUDINAL AXIS THEREOF, SAID RADIO FREQUENCY FIELD IONIZING SAID GAS, AND MEANS FOR EXTRACTING TRANSVERSE TO SAID RADIO FREQUENCY FIELD THE POSITIVE IONS CREATED BY SAID RADIO FREQUENCY FIELD. 